Ligand Redox Activity of Organonickel Radical Complexes Governed by the Geometry

Nickel-catalyzed cross-coupling reactions often employ bidentate π-acceptor N-ligands to facilitate radical pathways. This report presents the synthesis and characterization of a series of organonickel radical complexes supported by bidentate N-ligands, including bpy, phen, and pyrox, which are commonly proposed and observed intermediates in catalytic reactions. Through a comparison of relevant analogues, we have established an empirical rule governing the electronic structures of these nickel radical complexes. The N-ligands exhibit redox activity in four-coordinate, square-planar nickel radical complexes, leading to the observation of ligand-centered radicals. In contrast, these ligands do not display redox activity when supporting three-coordinate, trigonal planar nickel radical complexes, which are better described as nickel-centered radicals. This trend holds true irrespective of the nature of the actor ligands. These results provide insights into the beneficial effect of coordinating salt additives and solvents in stabilizing nickel radical intermediates during catalytic reactions by modulating the redox activity of the ligands. Understanding the electronic structures of these active intermediates can contribute to the development and optimization of nickel catalysts for cross-coupling reactions.


■ INTRODUCTION
−9 Radical pathways involving nickel(I) and (III) intermediates for generating and regulating carbon radicals have paved the way for the application of nickel catalysts in stereo-convergent coupling, 10 photoredox, and electrocatalytic reactions, 11 significantly broadening the scope and application of cross-coupling reactions.Notably, the crosselectrophile coupling reaction exemplifies the crucial role of distinct nickel(I)-halide 12 and nickel(I)-aryl 13 species in activating C(sp 2 ) and C(sp 3 ) electrophiles, respectively, through diverse mechanisms (Scheme 1). 14,15Recent research has also revealed the formation of nickel(III) intermediates through the rapid capture of carbon radicals by nickel(II) complexes, with a barrier of 7−9 kcal/mol, enabling efficient reductive elimination. 16he development of transition metal-catalyzed reactions relies on the careful selection of appropriate ligands to enhance reactivity and stabilize intermediates.In many cross-coupling reactions, strong σ-donor and π-acceptor ligands, such as terpyridine (terpy), 17 bipyridine (bpy), 18−20 1,10-phenanthroline (phen), 18 pyridine-bis(oxazoline) (pybox), 21 α-diimine, 22 and pyridine-oxazoline (pyrox), 23 have proven instrumental.These ligands can be redox-active, allowing for the stabilization of low-valent nickel radical species by delocalizing the unpaired electron into the π* orbital of the ligand. 24This redox activity plays a pivotal role in promoting radical pathways and differentiates the reactivity of nickel catalysts from the traditional two-electron pathways mediated by palladium catalysts.The lack of redox activity in ligands can lead to significant differences in the redox potentials of the nickel intermediates, resulting in notable changes in the reaction mechanism. 25Therefore, it is crucial to characterize the redox activity of ligands with respect to various nickel intermediates for comprehensive understanding of the reaction mechanism and informing catalyst optimization.
Moreover, the presence of anionic ligands can influence the speciation, complexation, and electronic structure of catalytic nickel intermediates, adding complexity to the catalyst effect and the mechanistic profile (Scheme 1).Additives such as MgCl 2 and KI have proven crucial in promoting catalytic reactions, such as cross-electrophile coupling reactions. 14hile recent studies have started to elucidate the beneficial effects of additives on these reactions, 26 the influence of anionic ligands on the electronic structure of nickel radical species and, consequently, the stability and reactivity of nickel intermediates remains unexplored.
A common perception is that the redox activity of a ligand is influenced by the nature of the actor ligands.Strong-field ligands, such as alkyl and aryl groups, can promote redox activity of the auxiliary ligands, whereas weak-field ligands, such as halides, are expected to result in nonredox activity of the auxiliary ligands. 31,32Data summarized in Scheme 2, however, reveal a potential correlation between the redox activity of a ligand and the coordination number and geometry of the complexes.Four-coordinate, square-planar complexes demonstrate ligand redox activity, whereas the same ligands in three-coordinate complexes do not display redox activity.Nevertheless, several classes of nickel radical complexes lack To address this knowledge gap, this report presents a synthesis and spectroscopic study focused on the electronic structures of nickel radical complexes.By completing the missing pieces and expanding the series of organonickel radical complexes, we provide compelling evidence that establishes the correlation between the redox activity of the ligand and the geometry and coordination number of nickel radical complexes bearing bidentate N-ligands.This finding is significant as the coordination number and geometry of nickel complexes can be modulated by the selection of reaction conditions and additives.Insights into the electronic structures of the active intermediates will inform the development and optimization of catalysts.
We then investigated the electronic structure of radical (bpy)Ni-aryl derivatives.Previous electrochemical and spectrochemical studies suggested the formation of four-coordi- The reduction of 16 with KC 8 at −78 °C in Et 2 O generated an olive-green species 17 (Scheme 4).The EPR spectrum of 17 at 30 K exhibited a rhombic signal with g values of [2.263, 2.086, 2.050].In contrast, when the reduction by KC 8 was conducted in the presence of THF as a cosolvent, the resulting product 18 appeared as a darker forest green solution compared to 17.The EPR spectrum of 18 displayed an isotropic signal with a g iso value of 2.004 at 100 K.These distinct EPR data suggest that 17 corresponds to a nickelcentered radical, while 18 is a ligand-centered radical.
To further investigate the electronic structures of (dtbpy)Niaryl radical complexes, we employed a bulkier aryl ligand, 2,6bis-Dipp-phenyl (Dipp*), to stabilize the complexes without introducing substituents on dtbpy (Scheme 5).We successfully synthesized Our attempt to abstract chloride from 19 using 1 equiv of NaBAr 24 F resulted in the formation of a π-allyl Ni(II) cationic species 22.The structure of 22 was determined through singlecrystal X-ray crystallography, revealing that Dipp* underwent a rearrangement to form η 3 -coordination, possibly via a 1,5-H shift (cf. Figure S69).Reduction of 22 with KC 8 generated complex 23.The isotropic EPR signal of 23, with a g iso value of 2.003, led us to assign its electronic structure as a π-allyl Ni(II) complex coordinated by [dtbpy] •− .
We also investigated the electronic structure of (bpy)Nidialkyl radical complexes by conducting the reduction of their Ni(II) analogues (Scheme 6).By adding 2 equiv of TMSCH 2 Li to (dtbpy)NiBr 2 24 in a mixture of THF and pentane, we obtained (dtbpy)Ni(CH 2 TMS) 2 25 in a yield of 62% as a dark green solid.Further reduction of 25 with KC 8 in THF in the presence of 18-crown-6 furnished [(dtbpy)Ni-(CH 2 TMS) 2 ] -[18-crown-6(K)] + 26 as a dark red crystalline solid.The single-crystal X-ray structure of 26 displayed a square-planar geometry, reminiscent of a previously reported analogous complex, [( tBu pyrox)Ni(CH 2 TMS) 2 ] -[18-crown-6(K)] + 9. 23 The EPR spectrum of complex 26 exhibited an isotropic signal with a g iso value of 2.006 and showed hyperfine splitting, which was attributed to the interaction of the radical with two N-and two H-atoms.Therefore, we assigned the electronic structure of complex 26 as a ligand-centered radical coordinated to a Ni(II) center.
The collective analysis of EPR data and single-crystal X-ray diffraction crystallography allowed for the unambiguous assignment of the electronic structures of a series of (bpy)Ni radical complexes (Table 1

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Phen-Ligated Nickel Radical Complexes.Phen has commonly been considered as a bpy analogue in the advancement of catalytic reactions.However, the conjugation between the two pyridine rings in phen can substantially influence its redox potentials. 34While a few examples of three-coordinate phen-ligated Ni(I) complexes have been documented, 12,18,30 our investigation focused on the relatively unexplored four-coordinate phen radical complexes (Scheme 7).The reduction of (phen)NiBr 2 27 with KC 8 in the presence of dibenzo-18-crown-6 (DB18C6) yielded a dark violet species, 28.The EPR spectrum displayed the presence of an organic radical with a g iso value of 2.001 and hyperfine couplings corresponding to the interaction of the radical with two N-and one H-atoms.Despite some similarities with [phen] •− , the EPR signal of 28 exhibited more pronounced line-broadening, which can be attributed to the quadrupole moment of bromide, leading to faster relaxation. 35The EPR signal, including both intensity and hyperfine pattern, was significantly influenced by the nature of the crown ether, suggesting the association of [crown]K + with 28 (cf.Figures S58 and S59).The stability of 28 closely correlated with the nature of the halide, as the complex rapidly decomposed when bromide was replaced with chloride.Unfortunately, our attempts to obtain a single crystal of 28 were unsuccessful due to its instability.Based on the analysis of the available evidence, we tentatively propose the structure of 28 as a fourcoordinate complex, [K(DB18C6)] + [(phen)NiBr 2 ] − , where the Ni(II) complex is ligated with a phen-centered radical.
The transmetalation of 27 with TMSCH 2 Li resulted in the formation of (phen)Ni(CH 2 TMS) 2 29, a navy blue solid with  Pyrox-Ligated Nickel Radical Complexes.We conducted further investigations on tri-coordinate (pyrox)Ni radical complexes, which have received limited attention in previous studies. 23Our previous attempts in their synthesis encountered challenges due to the relatively facile dissociation of pyrox compared to bpy or phen, resulting in the formation of undesired (pyrox) 2 Ni complexes.To stabilize the molecules, we introduced di-benzyl substituents on the oxazoline and a mesityl group on the pyridine to increase the steric bulk of the pyrox ligand (Scheme 8A).By comproprotionation of (pyrox)NiCl  23 In contrast, the C ox −N ox and C py −N py bonds of 10, a pyrox radical anion, are notably longer, while the C ox −C py bond is shorter compared to those in 32.The EPR spectrum of 32 exhibited a rhombic signal at 30 K, with g values of [2.448, 2.138, 2.070].By comparing the bond lengths of 32 to ( tBu pyrox)Ni(Dipp) 2 and 10 and considering the observation of a nickel(I) radical in EPR spectroscopy, we concluded that 32 is best described as a nickel-centered radical with a non-redox-active pyrox ligand.
Lastly, we employed NaBAr 24 F to abstract the bromide from ( tBu pyrox)Ni(Dipp)(Br) 33, resulting in the formation of complex 34 using a similar synthetic protocol as in the preparation of 16 (Scheme 8B).The reaction led to a color change from maroon to amber.The 1 H NMR spectra of 34 exhibited upfield shifts compared to those of 33 (Figure S45).HRMS analysis of 34 revealed an exact mass of 424.2081 (M-BAr 24 F + H), indicating the absence of the bromide ion.In contrast, HRMS of 33 displayed a mass of 525.1379 (M + Na).Based on these findings, we assigned complex 34 as the cationic [( tBu pyrox)Ni(Dipp)] + [BAr 24 F ] − .Further reduction of 34 with KC 8 yielded a dark green solution, and the corresponding EPR spectrum displayed a rhombic signal with g values of [2.308, 2.101, 2.057].We assigned the structure of 35 to a three-coordinate [( tBu pyrox)Ni(Dipp)] complex as a nickel-centered radical.
DFT Calculations.We performed density functional theory (DFT) calculations on the electronic structures of the nickel radical complexes described in this study.The geometry optimization was found to be highly sensitive to the functional and the basis set.While the commonly used basis set, (U)B3LYP-D3/def2-TZVPP, was successful in reproducing the experimental geometries of [NiR 2 ] − and NiX complexes, it performed poorly with Ni−Ar and [NiX] 2 complexes.As a result, we applied the combination of (U)B3LYP//m6-31g* for the Ni−Ar complexes. 31In general, the electronic structures obtained from DFT calculations are consistent with experimental data.The four-coordinate complexes were computed to be square planar with highly delocalized spin density on the ligands, and the ligand C�N bonds were elongated (Figures S75 and S77−S80).Among the threecoordinate Ni(I) complexes, the Ni(I)-halide and Ni(I)-phenyl complexes were computed to be trigonal planar with localized spin density on the Ni centers (Figures S73, S74, S76, S81, and S82).

■ DISCUSSION
In this study, we synthesized a series of nickel radical complexes and characterized their electronic structures using NMR, EPR, mass spectroscopy, and X-ray crystallography.These new complexes highlighted in red in Scheme 9A, complete the range of nickel radical analogues, allowing us to draw empirical conclusions regarding the correlation between coordination geometry and redox activity of the ligand.In general, four-coordinate nickel radical complexes adopting a square-planar geometry can be best described as low-spin nickel(II) complexes coordinated with ligand radical anions.In contrast, three-coordinate nickel-halide, -alkyl, and -aryl complexes adopt trigonal planar or distorted trigonal planar geometries.These molecules are characterized as nickel(I) centers coordinated with non-redox-active ligands.This pattern remains consistent regardless of the identity of the X-ligands.

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While tetrahedral nickel radical complexes are beyond the scope of this study, several such molecules have been reported within the α-diimine ligand framework, including [(αdiimine)Ni(μ-H)] 2 and [(α-diimine)Ni(μ-halide)] 2 complexes. 22In contrast to square-planar nickel complexes, most of these tetrahedral molecules are characterized as nickelcentered radicals, where the α-diimine ligand does not exhibit redox activity.
Based on the data presented in this report and literature precedents, we propose that the redox activity of bidentate Nligands in low-valent organonickel radical complexes is dependent on the geometry and coordination number.The redox activity of the ligand is determined by the relative energy levels of the unfilled d orbital and the π* orbital of the ligand (Scheme 9B).Molecules of trigonal planar and tetrahedral geometries have relatively low-energy antibonding d orbitals, which can be lower than the π* orbital of the ligand.Consequently, the unpaired electron tends to occupy the d orbital, resulting in a d9 electronic configuration.In contrast, square-planar complexes have high-energy d x 2 −y 2 orbitals.In this geometry, the unpaired electron prefers to occupy the π* orbital of the ligand, leading to a nickel(II) center coordinated with a radical anion ligand.
One of the most compelling pieces of evidence supporting this hypothesis is the contrasting electronic structures of 17 and 18 (Scheme 4).Introducing a coordinating solvent to shift the coordination number from three to four resulted in changes in both the redox activity of the ligand and the oxidation state of the nickel center.These results have significant implications for catalyst optimization.In nickelcatalyzed cross-coupling reactions, various additives such as MgCl 2 and KI, have been extensively employed.Besides their role in modulating the speciation of active nickel catalysts, which has recently been elucidated, 26 the presence of coordinating anions may also play a crucial role in stabilizing nickel(I) intermediates and tuning the redox potentials by inducing ligand redox activity.Furthermore, our data suggest that the use of coordinating solvents may benefit the reaction by exerting a similar stabilization effect.
An exception to this postulate has been reported before: a square-planar (terpy)Ni-methyl complex exhibits a ligandcentered radical, whereas square-planar (terpy)Ni-bromide 31 and (terpy)Ni-phenolate 32 complexes display metal-centered radicals.This observation may be attributed to various factors, including a relatively low-lying d x2-y2 orbital with weak-field ligands, the rigid geometry of terpy that precludes alternative geometries other than square-planar, or potential π-stacking effects caused by the planar terpy ligand.Ongoing research endeavors aim to further elucidate the underlying factors governing ligand redox activity of tridentate ligands.

■ CONCLUSIONS
We have synthesized and characterized a series of nickel radical complexes that hold significant catalytic relevance.This comprehensive collection of complexes includes various (bpy), (phen), and (pyrox)nickel radical analogues, which enable us to establish a clear correlation between the coordination geometry and the redox activity of the ligands.Specifically, the four-coordinate square-planar nickel radical complexes are low-spin nickel(II) centers coordinated with radical anion ligands.In contrast, the three-coordinate nickel radical complexes exhibit a trigonal planar geometry, featuring nickel(I) centers coordinated with ligands that do not display redox activity.This trend remains consistent regardless of the identity of the X-ligands.These findings provide an account for the important role of coordinating salt additives and solvents in modulating the stability and redox potentials of nickel intermediates by altering the coordination number and inducing ligand redox activity.This understanding of the relationship between the coordination environment and ligand redox properties is crucial for the design and optimization of catalysts in the development of nickel-catalyzed cross-coupling reactions.

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Scheme 2 .
Scheme 2. Electronic Structures of Precedent and Uncharacterized Organonickel Radical Complexes a

Scheme 3 .
Scheme 3. Synthesis and Characterization of Three-Coordinate (bpy)Ni(I)-Halide and -Alkyl Complexes Journal of the American Chemical Society (dtbpy)Ni(Dipp*)Cl 19 by adding (Dipp*)Li to [(dtbpy)Ni(μ-Cl)] 2 . 12,33Reduction of 19 with KC 8 yielded intermediate 20, as a forest green solution.The EPR spectrum of 20 exhibited an isotropic signal at 295 K with a g iso value of 2.007, indicating a ligand-centered radical.The observed hyperfine splitting was attributed to the coupling of the radical with two N-and two H-atoms on the dtbpy ligand.Further crystallization of a solution of 20 in pentane led to the formation of a new species 21.X-ray crystallography established that 21 was a tri-coordinate (dtbpy)Ni(Dipp*) complex with a trigonal planar geometry (Scheme 5).The dihedral angles of N1−N2−Ni−C3 in the two crystallographically unique molecules were measured to be 171.12°and158.37°.The EPR spectrum of 21 at 30 K exhibited a rhombic signal with g values of [2.585, 2.139, 2.070].Based on the structure of 21 and a comparison of the EPR spectra of 20 and 21, we assigned the structure of 20 as the (dtbpy)Ni(Dipp*)Cl radical anion, featuring a ligand-centered radical coordinated to a Ni(II) center.
Scheme 6. Electronic Structure of the Four-Coordinate (dtbpy)Ni-Dialkyl Radical Anion Complex

Scheme 7 .
Scheme 7. Synthesis and Electronic Structures of Four-Coordinate (phen)Nickel Radical Complexes Scheme 8. Synthesis and Characterization of Three-Coordinate (pyrox)Ni Radical Complexes Journal of the American Chemical Society

Scheme 9 .
Scheme 9. Electronic Structures of Nickel Radical Complexes (A) and the Comparison of SOMO Energy Levels in Different Geometries (B)

Table 1 .
EPR and Single-Crystal X-ray Structure Parameters of (bpy)Nickel Complexes a Two crystallographically independent molecules.